1. Field of the Invention
The present invention relates to a compound semiconductor material and, more particularly, to a compound semiconductor material suitable for forming the active layer of a thin film transistor of an active matrix flat panel display.
2. Description of Related Art
Recently, the active matrix flat panel display has become a popular focus for research and development. In particular, the emphasis of that research and development is directed to the thin film transistor. In addition to that, the flat panel display has been developed to have a large active area, a low price, and a high resolution. Currently, the thin film transistors (TFTs) are classified as the amorphous silicon TFTs and the low temperature polycrystalline silicon TFTs, both of which need to undergo the vacuum evaporation process and the photolithography process, and thus have a high manufacturing cost. Lately, a method for manufacturing a thin film transistor using a group II-VI compound semiconductor by a solution process is disclosed. This method requires simple equipment, and has low cost and a fast processing speed.
However, take the manufacturing of ZnO transistors by the Sol-gel process for example. Although the Sol-gel process is simple and the materials used therein have a low toxicity, the characteristics of the transistors produced are not satisfactory. For instance, the current on/off ratio obtained is 103, as shown in FIG. 1. Besides, there is research to manufacture a group II-VI compound, such as CdSe or CdS, transistors by a Chemical Bath Deposition (CBD) process. As shown in FIG. 2, the mobility of CdSe transistors reaches 15 cm2/Vs, the current on/off ratio obtained is 107, and the critical voltage is 3.5 V. Similarly, the mobility of CdS transistors reaches 1 cm2/Vs, the current on/off ratio obtained is 106, and the critical voltage is 2.6 V, as shown in FIG. 3. Although they both have good electrical characteristics, the precursors of CdSe and CdS are heavy metals that are quite toxic. Therefore, it is not feasible to mass-produce those transistors due to the problems of safety and environmental protection.
In the prior arts, the doped ZnO materials are mainly applied to the manufacturing of fluorescent films, and have not been used in the active layer of transistors yet. Therefore, it is desirable to provide a compound semiconductor material and a method for forming an active layer of a thin film transistor device to mitigate and/or obviate the aforementioned problems.